Postage meters (hereinafter sometimes designated simply as "meters") are well-known devices for imprinting postage impressions of desired value either on a gummed tape or directly on an article to be mailed, thereby obviating the need to use postage stamps. Due to their convenience and flexibility, meters have found widespread use in commerce.
A postage meter normally includes a postage selection mechanism, a postage printing mechanism, and a plurality of internal registers for maintaining accounting information. The internal registers most commonly contain numerical values representative of the total postage paid for (control total), the total postage printed (ascending balance or ascending register), and the total postage remaining (descending balance or descending register). The information contained in the internal registers is redundant, since the ascending balance and descending balance normally sum to the control total.
Prior to using the meter a user must buy from a postal service employee a fixed amount of postage. (In this connection, the term "postal service" may refer to either a public or private mail carrying entity.) The postal service employee alters the contents of the internal registers to reflect the amount of postage paid by increasing the control total and the descending balance by this amount. To use the meter, the user first selects a value of postage to be printed, and then activates the printing mechanism. The meter may be used until the descending balance reaches a predetermined minimum (e.g., until the postage paid for has been exhausted or has reached a minimum threshold value).
It can immediately be seen that postage meters are subject to stringent security requirements to ensure that all postage actually printed has been paid for. Thus, the level of security can be measured by the difficulty of activating the meter'printing mechanism without correspondingly updating the accounting registers within the meter, and also by the difficulty of altering or losing the meter register values, whether intentionally, inadvertently, or accidentally. To this end, the print mechanism and the accounting registers are located within a secure housing, and access thereto is restricted to postal service employees.
Postage meters have traditionally been essentially mechanical devices whose mechanical design is relatively complicated due to the need to correlate operation of the postage selection mechanism, the postage printing mechanism, and the registers. In parcticular, the print mechanism must print a postage value corresponding to the value set by the user, and the appropriate internal registers must be changed by this amount. Moreover, the meter must be interlocked to disable the print mechanism when the descending balance reaches the predetermined minimum level, and to prevent more than a single printing impression from being made during a cycle of the printing mechanism. Mechanisms capable of performing these functions, of necessity, contain a large number of mechanical parts, and therefore require considerable periodic maintenance. While several decades of experience have resulted in the design and implementation of acceptably reliable mechanical postage meters, such devices have still tended to be expensive, heavy, bulky, and slow.
Recent advances in the electronic arts have suggested the desirability of replacing many of the mechanical components in a postage meter with electronic components. Thus, it is known in the prior art to provide a first-generation electronic postage meter employing discrete logic components. Such a meter is shown in U.S. Pat. No. 3,938,095 to check, Jr. et al.
By their nature, electronic postage meters rely heavily on continuous electric power during operation. However, frequent power loss of either a momentary or prolonged nature is to be expected. While power loss is not a particularly significant event for mechanical meters, it poses two distinct threats to the security of electronic postage meters. First, power loss presents a threat to the integrity of the register data which is typically stored in electronic memory units, since most electronic memories are volatile devices (i.e., they require continuous electric power to maintain their contents). This is to be contrasted with mechanical registers which are inherently nonvolatile devices. Second, the various correlation and interlocking functions are performed by electronic logic components, the performance of which can become unpredictable during a low power condition. Since this could lead to improper updating of registers, and the like, there must be provided a reliable mechanism wherein the electronic circuitry inhibits meter functioning when a low power condition is sensed. Moreover, this inhibiting must occur before the power falls to a level at which the electronic circuitry becomes unreliable.
In addition to the security requirements discussed above, a second requirement of postage meters, called "fault tolerance", comes into play when mechanical registers and other functions are replaced by electronic components. Fault tolerance refers to the meter's ability to maintain security in view of individual component failure. A postage meter is likely to be used in a variety of settings that may subject the components to environmental rigors such as mechanical shock, stray electric fields, and wide temperature variations, any of which may cause an electronic, mechanical, or electromechanical component to fail.
It is apparent that the large number of components in the first-generation electronic postage meters employing discrete logic elements (i.e., transistors, diodes, etc.) tends to render such meters insufficiently reliable for postal service approval. A further difficulty with electronic postage meters employing discrete logic components is that the features and capabilities of the meter cannot be altered easily once the meter is constructed. Thus, like their mechanical counterparts, such meters cannot readily be adapted to new applications.
In recognition of the above problems, first-generation electronic postage meters employing discrete logic have given way to second-generation electronic postage meters employing large scale integration microcomputer architecture. An example of such a second-generation stand-alone postage meter is disclosed in U.S. Pat. No. 3,978,457 to Check, Jr. et al., employing a microcomputer system which monitors the printing and other functions of the meter, and which supervises and maintains the required accounting information. A microcomputerized postage meter contains a smaller number of components than its discrete component counterpart, and is therefore likely to have improved fault tolerance characteristics. The fault tolerance can be further enhanced by the capability possessed by such a meter of verifying its own functions. Nevertheless, fault tolerance remains a potentially vexing problem because the very components that are used to check for failure are themselves subject to failure.
In spite of the numerous potential advantages of electronic postage meters over their mechanical predecessors, and further in spite of the expanded capability of microcomputerized postage meters, efforts to design an electronic postage meter having sufficiently high levels of security and fault tolerance to obtain postal service approval have been generally unsuccessful to date.